Absolute Altitude (AGL)

Absolute Altitude (Above Ground Level) is the vertical distance between an aircraft or any object and the ground surface directly beneath it. Unlike Mean Sea Level (MSL), which is based on the average elevation relative to sea level, absolute altitude varies depending on local surface conditions. The AGL value is calculated based on the elevation of the ground surface at the aircraft’s current location and therefore changes along the flight path as terrain elevation varies.

Measurement Methods

AGL is not measured directly using pressure-based altimeters. Such devices provide height information relative to MSL based on changes in atmospheric pressure. To determine absolute altitude, radar altimeters or radio altimeters are used. These systems emit radio waves toward the ground beneath the aircraft and measure the time it takes for the signals to return, calculating the distance from this time delay. This provides a direct measurement of the actual distance to the ground surface. In particular during descent and landing phases, this type of measurement offers pilots a more accurate and safer reference.

Applications

The concept of absolute altitude is critical for flight safety, approach and landing procedures, obstacle clearance safety, and ground collision risk analysis. In meteorological reports, cloud base height is typically given in AGL units. This enables pilots to directly assess visibility and cloud ceiling relative to ground level. Additionally, in air traffic control services, flight is permitted only within specified altitude limits above ground in certain areas; these restrictions are defined using AGL as the reference.

Relationship Between AGL and MSL

MSL is an altitude measured relative to the average sea level surface. AGL is derived by subtracting the elevation of the ground surface from the MSL value. Consequently, two aircraft flying at the same MSL altitude may have different AGL values depending on the elevation of the terrain beneath them. This difference is particularly pronounced between high mountainous regions and areas near sea level. The relationship is expressed by the formula: AGL = MSL − Ground Elevation.

Application and Safety

AGL measurement is a critical parameter in instrument approach and automatic landing systems. These systems ensure the aircraft maintains safe altitudes relative to the runway threshold. In low-visibility approaches, radar altimeter data is displayed on pilot instruments to continuously monitor the risk of ground contact. This measurement is recognized as one of the fundamental elements of navigation and landing safety in aviation.

Terms and Symbols

In international aviation terminology, AGL is the abbreviation for “Above Ground Level” and is translated into Turkish as “Mutlak İrtifa.” This concept is represented in flight charts and operational documents using the unit “height.” In contrast, values expressed relative to MSL are designated as “altitude.” This distinction ensures a consistent terminological standard in air traffic management, mapping, and flight planning.